Conductor for underground transmission of electric power

ABSTRACT

A conductor adapted for underground transmission of alternating current consisting of a plurality of separate strands includes strands of copper clad with aluminum, the aluminum being about 10 percent to 20 percent of the cross-sectional area of the conductor. The conductor may be made up of aluminum-clad copper strands intermingled with all-copper strands, or strands of copper having oil-impervious coating, the strands being arranged so that the surface of each copper, or coated copper strand, is only in contact with the surface of an aluminum-clad strand. The strands are preferably grouped in segments and helically wound in each segment.

' United States Patent [.72] Inventor Carlos Katz Bayonne, NJ.

[21] Appl. No. 5.238

[22 Filed Jan. 23,1970

[45] Patented Aug. 10, 1971 [73] Assignee GeneralCableCorporation New York, N.Y.

[54] CONDUCTOR FOR UNDERGROUND TRANSMISSION OF ELECTRIC POWER 8 Claims, 3 Drawing Figs.

[52] 11.8. C1 174/119, 174/113,174/126 [51] 1nt.C1 1. 1101b 7/00 [50] Field ofSearch 174/113, 114,119, 126,128, 130.25, 26, 1 10.1; 29/191.6, 193, 199

| 5 6] References Cited UNITED STATES PATENTS 1,904,162 4/1933 Milliken 174/114 El'C'TP/C'ALLY 2,125,869 8/1938 Atkinson ..1.74/114(.21UX

2,187,213 1/1940 Milliken.. 174/114 2,972,658 2/1961 Lapsley 174/114 FOREIGN PATENTS 1,170,845 1/1959 France 174/1262 Primary Examiner- Lewis H Myers Assistant E.tam1'nerA. T. Gnmley Attorney-Sandoe, Hopgood & Calimafde ABSTRACT: A conductor adapted for underground transmission of alternating current consisting of a plurality of separate strands includes strands of copper clad with aluminum, the aluminum being about 10 percent to 20 percent of the crosssectional area of the conductor. The conductor may be made up of aluminum-clad copper strands intermingled with allcopper strands, or strands of copper having oil-impervious coating, the strands being arranged so that the surface of each copper, or coated copper strand, is only in contact with the surface of an aluminum-clad strand. The strands are preferably grouped in segments and helically wound in each segment.

(IA/SULA TIA/6 TAPE PATENIEDAusmmn 3593 99 ELECTRIC/ILL) GOA/00C 77 V6 TAPE f1 INVENTOR CARLOS KATZ r WM.

ATTORNE YS.

CONDUCTOR FOR UNDERGROUND TRANSMISSION OF ELECTRIC POWER BACKGROUND OF THE INVENTION The present invention is an improved conductor construction for reducing power losses and increasingthe load-carrying capacity of large-size conductors used for transmission of AC electric power. The conductor construction of this invention is particularly adapted for underground cable systems.

The load capability of underground cable systems for transmission of AC electric power depends primarily upon: losses in the cable shield, losses in the duct in which the conductor is installed, the thermal resistance of the cable system and on the electrical resistance of the conductor.

In order to minimize such losses and obtain larger load capabilities in conductors which have relatively large cross sections in excess of 1,000 MCM, for example the conductors are customarily formed of a plurality of conductor strands arranged in segments (generally four) with the strands in each segment twisted helically around each other. The segments may be insulated from each other by a film of dielectric material, such as a cellulosic paper or a synthetic resin plastic.

In AC transmission the resistance is increased, and hence the load-carrying capacity of a cable system is reduced by the distortion of the relative density of the current over the cross section of the conductor. One of these distorting effects is the skin effect, which is the tendency of the AC current to concentrate at the outside of the conductor or insulated segment. The magnitude of the skin effect depends on the resistivity of the conductor material, the frequency of the alternating current, the arrangement of the strands helically wound in segments, and/or concentrically wound, and on the degree of contact resistance among the strands. The distribution of the current density over the cross section of a conductor is additionally distorted by the proximity of other conductors, for example, in a pipe-type installation wherein a number of cables or conductors are closely adjacent one another in a pipe or duct.

The conducting materials most widely used for the strands of conductors for the underground transmission of electric power are copper and aluminum. Aluminum has larger electrical contact resistance between adjacent strands than copper, but copper is the better conductor.

In accordance with the present invention the load-carrying capability of a stranded segment conductor is increased by reducing the skin and proximity effects by using at least some strands of copper clad with aluminum therein.

DESCRIPTION OF THE DRAWINGS The invention is described below in detail with respect to the accompanying drawings in which:

FIG. I is an isometric view showing an aluminum-clad copper strand as used in the present invention, showing it with a portion of the cladding stripped away;

FIG. 2 is an isometric view looking at the end ofa stranded segmented conductor in accordance with the invention, with part of the outer wrapping broken away to reveal the individual strands;

FIG. 3 is an end view ol'a stranded segmented conductor illustrating a varied form ofthe invention.

Referring to the drawings, a conductor in accordance with the invention includes at least some strands made up of a copper wire 11 clad with an exterior sheath 12 of aluminum metallurgically bonded to the copper.

The aluminum cladding may be applied on the copper wire in any suitable manner. One method is to coat the copper wire with silver, insert the coated wire into a tube of aluminum or extrude a layer of aluminum directly over the silver coating, and then rolling or drawing the composite strand to squeeze the aluminum, silver and copper into intimate contact to provide a complete metallurgical bond by bonding the copper and silver and the silver and aluminum, which respectively bond metallurgically together.

In the embodiment shown in FIG. 2 all the strands of the conductor are aluminum-clad copper strands 10. As shown, they are in four segments, with the strands in each segment being helically wound therein. Opposite segments 13 are insulated and this insulates all four segments from one another. The uninsulated segments 13a are preferably slightly larger than the insulated ones to compensate for the thickness of the insulation and thus allow for the manufacture of a perfectly round table. Only one segment 13 is illustrated in detail, and it is an insulated segment, it being understood that the other three segments 13, and 13a are similar therewith, except that the segments between the insulated segments do not need to have insulation. The segments are bound together to form a composite conductor by an overwrapping of an electrically conductive tape 14 which may be a conventional material for this purpose, such as bronze tape intercalated with paper or other metallic tape where the tape has sufficient strength not to require the intercalated paper, and which serves as a mechanical binder and as a conductive sheath. As mentioned above, the segments 13 may be insulated from one another by a suitable dielectric material on opposite strands, such as by insulating tape 16 shown in the drawings.

For the purpose of this invention the aluminum cladding 12 preferably comprises from about 10 percent to 20 percent of the cross-sectional area of the strand l0 and most suitably comprises about.l0 percent as shown by the tables set forth below. 1

The following tables 1 and 2 show the relative current-carrying capacity of cables in which the conductor material was copper, aluminum, enamel-coated copper, and copper wires clad with aluminum wherein the aluminum comprised respectively 20 percent and It) percent of the cross-sectional area. Table I shows test results. Table 2 shows calculated results. The cables tested were one conductor, segmented, 2,000 MCM, I38 kv., HPOF cables and were tested at room temperature. In table 1 the AC conductor resistance (R was measured in air (tested in triangular formation and again in cradle formation.

The current-carrying capacity of the conductors in pipetype cable systems is given by the equation I: ATc

1 R 1+Yc)Rcs where: l= current carrying capacity of the conductor in amperes,

AT== maximum rise of temperature allowable at the conductor in C.,

1.7R,,,( I+Yc) effective electrical resistance of the conductor with the cables installed in a steel pipe in microhms per foot.

R,, direct current resistance ofthe conductor in microhms per foot.

Y0= increase in conductor resistance due to alternating current effects expressed as a fraction of the DC resistance, and

R,.,,== effective thermal resistance ol the thermal circuit in thermal ohms-foot.

Since ATc and R can be considered as constants for particular conditions of conductor temperature, dielectric, duct and cable installation environment, a way of increasing the current-carrying capacity of the conductor, without changing the conductor size, is by decreasing its effective resistance. This is accomplished in accordance with the present invention by a conductor made totally or partially of aluminum-clad copper strands.

The cables tested were one conductor segmented, 2,000 MCM, I38 kv., HPOF cables, and were tested at room temperature.

For table 1 the AC resistance of each conductor (R,,,.) was tested in air with the cables in triangular and in cradle formation and is given by R l=Ycl where Ycis a factor due to skin and proximity effects in the conductor.

TABLE NO. 1

llC, segmental, 2,000 MGM, 138 k\'., IIPOF Cables Triangular formation C radlo formation in air in air dr, un 7 ue,

microhm/ microhrn/ ruicrohm/ Conductor material It. 1+Yc 1+ Y ft.

Copper 5. 42 1. 26 6. 82 1.23 6. 68 Aluminum 8.80 1.02 8. 97 Enamel coated copper (92% Cu., 8% enaruel) 5.86 1. 05 6.15 1.04 6.10

' Based on values measured on a 2,250 MOM conductor and extrapolated for a 2,000 MOM conductor.

As indicated in this table 1. the effective resistance of plain copper conductors is apparently largely increased by skin and proximity effects. but this is not the case with the aluminum conductor or the enamel-coated copper conductor. The smaller skin and proximity effects in these latter conductors are due to the better current density distribution over their cross sections; in the case of the aluminum conductor the improved current distribution is apparently a consequence of high electrical contact resistance among the strands. in the case ofthe enamel-coated copper conductor it is apparently a consequence of the insulating effect among the strands provided by the enamel.

Table 2 shows the advantage of the aluminum-clad copper conductor over copper and over all-aluminum conductors. For table 2 the current-carrying capacity as given by the above equation was obtained by the equation:

in which p=l /R,, conductivity.

By assuming similar conductor size, dielectric, installation conditions and environments, the latter equation was rewritten:

l=Kw p/ (Il-Yc) and the values for current-carrying capacities were then calculated.

TABLE NO. 2

l/C, 2000MCM. Segmental. 138 kV, HPOF Cable Triangular Formation in Air ('urrent Carrying Taking into consideration that for cradle formation the effective resistance l+lc) is generally somewhat smaller than for triangular formation the corresponding current-carrying capacity will be slightly larger. However, the percent increase or decrease in current-carrying capacity with respectto that calculated for the triangular formation will be negligible.

At first glance the last column of table no. 2 is somewhat misleading. The indicated 6 or 8 percent increase in currentcarrying capacity in reality represents a to percent of the total possible increase which would be the case if the AC resistance would be the same as the DC resistance. The above part of a three-cable system in an 8 inch steel pipe is 1.91 times the DC resistance.

Table No. 2 thus indicates that segmented cable conductors made with aluminum-clad copper strands, where the aluminum comprises the outer surface of each strand in an amount of 10 to 20 percent of the total cross section, will have an 8 to 6 percent larger current carrying capacity than a conductor of similar size made with plain copper strands and about 19 to 21 percent larger load capability than a conductor of similar size made with aluminum strands.

The aluminum-clad copper strands 10 can be used in conductors of all shapes and sizes, but their main application will be in pipe-type cables, whereas in other systems economic considerations generally dictate the need of minimum losses and maximum load capabilities.

FIG. 3 shows a modified conductor construction of this invention wherein the strands in the segment 13 include allcopper strands 15 and aluminum-clad copper strands 10 intermingled and arranged relative to each other so that no two all-copper strands have their surfaces in contact with each other. Thus the high contact resistance of the aluminum is effective for reducing the skin and proximity effects and thereby increases the current-carrying capacity. For cables that are oil impregnated the all-copper strands 15 are suitably coated with a material, such as a lead-tin alloy, to provide a coating which is impervious to the oil to prevent the oil from coming in contact with the copper, and thus ensure a greater stability of the oil.

What I claim is:

l. A conductor adapted for use underground comprising a plurality of strands bound together, characterized in that at least some of said strands are copper clad with an outer layer of aluminum, said aluminum layer comprising from about 10 percent to about 20 percent of the cross-sectional area of each strand.

2. The conductor of claim 1 in which said aluminum cladding comprises about l0 percent of the total cross-sectional area of the strand.

3. The conductor of claim 1 in which others of said strands are all copper, the strands being arranged and bound together with the surfaces of the all-copper strands in contact only with surfaces of the aluminum-clad strands. I

4. The conductor of claim 3 which is adapted to be impregnated with insulating oil and in which the copper strands are coated with a material that is impervious to said oil for preventing the oil from contacting the copper.

5. The conductor of claim 4 in which said oil-impervious coatings are coatings of material from the group consisting of tin and lead-tin alloy.

6. The conductor of claim 1 in which the strands are grouped in segments bound together with an overlayer of electrically conducting tape.

7. The conductor of claim 6 in which the strands in each segment are helically wound therein.

8. The conductor of claim 6 in which the segments are insulated from each other by layers of electrical insulating material between adjacent segments.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 3,598 ,899 Dated August 1% 1971 IHVEMOHS) Larlos Katz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, TABLE NO. 2, should appear as shown below:

TABLE No.3" l/C 2000 MGM, Segmental, 138 kV, HPOF Cable Triangular Formation in Air Current Carrying Current Ca acit Conductor Conductivity Carrying Z, difference with Material 70 l Yc Capacity respect to copper Copper 100 1.26 8.9 k 0 Aluminum 60 1.02 7 7 k -l3 Al uminum (11 rd Copper (20%, Al.

80% Cu. 92 l 04 9.4 k 6 Aluminum Clad Copper (10% Al.

907o Cu. 96 l. 04 9. 6 k 8 Signed and sealed this 6th day of February 1973.,

(SEAL) Attest' IZUWARJ) M.FHE'I'CH1R ,.IR. ROBERT GOTTSCHALK Attost'ing ()[Ii cor Commissioner of Patents USCOMM-DC 603164 69 9 U S GOVEHNMENY Pmu'nur. nrnrr an. fii'lllixtl QM PC7-1050 (10-69) 

1. A conductor adapted for use underground comprising a plurality of strands bound together, characterized in that at least some of said strands are copper clad with an outer layer of aluminum, said aluminum layer comprising from about 10 percent to about 20 percent of the cross-sectional area of each strand.
 2. The conductor of claim 1 in which said aluminum cladding comprises about 10 percent of the total cross-sectional area of the strand.
 3. The conductor of claim 1 in which others of said strands are all copper, the strands being arranged and bound together with the surfaces of the all-copper strands in contact only with surfaces of the aluminum-clad strands.
 4. The conductor of claim 3 which is adapted to be impregnated with insulating oil and in which the copper strands are coated with a material that is impervious to said oil for preventing the oil from contacting the copper.
 5. The conductor of claim 4 in which said oil-impervious coatings are coatings of material from the group consisting of tin and lead-tin alloy.
 6. The conductor of claim 1 in which the strands are grouped in segments bound together with an overlayer of electrically conducting tape.
 7. The conductor of claim 6 in which the strands in each segment are helically wound therein.
 8. The conductor of claim 6 in which the segments are insulated from each other by layers of electrical insulating material between adjacent segments. 